Imaging-based barcode readers may be portable or stationary. A portable barcode reader is one that is adapted to be held in a user's hand and moved with respect to target indicia, such as a target barcode, to be read, that is, imaged and decoded. Stationary barcode readers are mounted in a fixed position, for example, relative to a point-of-sales counter and often referred to as a bi-optic scanner or bi-optic imager. Target objects, e.g., a product package that includes a target barcode, are moved or swiped past one of the one or more transparent windows and thereby pass within a field-of-view (“FOV”) of the stationary barcode readers. The barcode reader typically provides an audible and/or visual signal to indicate the target barcode has been successfully imaged and decoded.
A typical example where a stationary imaging-based barcode reader would be utilized includes a point of sale counter/cash register where customers pay for their purchases. The reader is typically enclosed in a housing that is installed in the counter and normally includes a vertically oriented transparent protection window and/or a horizontally oriented transparent protective window, either of which may be used for reading the target barcode affixed to the target object, i.e., the product or product's packaging for the product having the target barcode imprinted or affixed to it. The sales person (or customer in the case of self-service check out) sequentially presents each target object's barcode either to the vertically oriented window or the horizontally oriented window, whichever is more convenient given the specific size and shape of the target object and the position of the barcode on the target object.
The stationary and portable imaging-based barcode readers typically include at least one camera or scan engine and can include a plurality of scan engines located behind any number of protective windows. Each scan engine may have a different field-of-view from every other scan engine in the imaging-scanner system. While the fields-of-view may overlap to some degree, the effective or total field-of-view (“TFV”) of the multi-imaging scanner is increased by adding additional camera systems. Hence, the desirability of multicamera readers as compared to signal camera readers, which have a smaller effective field-of-view and require presentation of a target barcode to the reader in a very limited orientation to obtain a successful, decodable image, that is, an image of the target barcode that is decodable.
A typical scan engine comprises a plurality imaging pixel arrays or photosensitive elements such as charge coupled device (CCD) arrays and complementary metal oxide semiconductor (CMOS) arrays. The scan engine also typically includes an illumination system comprising light emitting diodes (LEDs) or cold cathode fluorescent lamp (CCFL) that direct illumination toward a target object, e.g., a target bar code. Light reflected from the target bar code is typically focused through a lens located near or on the scan engine within the imaging system such that the focused light is concentrated onto the pixel array or photosensitive elements. Thus, an image from a field-of-view of the focusing lens is focused on the pixel array. Periodically, the pixels of the array are sequentially read out by the scan engine, generating an analog signal representative of a captured image frame. The analog signal is amplified by a gain factor and the amplified analog signal is digitized by an analog-to-digital converter. Decoding circuitry of the imaging system processes the digitized signals and decodes the imaged bar code.
Such imaging systems as described above are usually very sensitive to stray lights from ambient and internal or external illumination. Ghost images and significant reductions in the image contrast undesirably occur as a result of stray light. Stray light is often considered as the least intuitive and most undesirable in any optical designs, and it is also usually very computationally intensive to model even using a modern computer. Moreover, it can be extremely costly to discover the presence of stray light at the end phase of any product design.